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Garabedian and colleagues' article (1) discusses potential bias in the instrumental variable analysis of observational studies. However, it did not mention an important type of instrumental variable approach for use with observational data, namely, the paired availability design (PAD) (2–4). Under the PAD, time period is the instrumental variable and the availability of new treatment for patients changes over time periods.

The PAD makes a key assumption that Garabedian and colleagues discuss: that the instrument affects the outcome only through treatment. The PAD also assumes stable preferences, namely, that treatment preferences do not change over time. For a fixed increase in time of availability in one period versus another, the stable preference assumption implies that the effect of treatment is always in the same direction, another key assumption that Garabedian and colleagues discuss. For a randomly occurring increase in time of availability in one period versus another, the stable preference assumption implies a certain type of randomness in the direction of treatment that yields an identical estimate of treatment effect as under a fixed increase in time of availability (4).